Vulcanized rubber products, such as discarded automobile tires can pose a difficult situation when they come to the end of their lifetime. On one hand, the tires can be put into landfill; however, they do not decompose rapidly. On the other hand, chemical decomposition of tires is possible, although the vulcanized rubber is a heavily crosslinked material which can only be broken down using highly aggressive chemical treatments. Thus, alternative solutions are required. One solution would be to recycle vulcanized rubber materials into products with general use in many applications.
One major use of recycled vulcanized rubber is the production of crumb rubber, which is composed of ground particles of rubber that has been separated from other tire components, such as steel and fluff, and has a size distribution in the range of 60-1000 μm. Crumb rubber currently sees major use in rubberized asphalt, and is used as a cushioning material in Astroturf and on playgrounds. For other applications, such as pigment and coating additives, the large particle size of crumb rubber limits its use. For such applications, a particle size of less than 100 μm is generally required and virtually all inclusions, such as steel or glass fibers, must be removed. If the mean particle size is less than 60 μm, it becomes possible to spray particulate rubber samples when using an appropriate paint system and binder.
Methods for decreasing the size of crumb rubber particles which involve cryogenically cooling the material to below or near its glass transition temperature and grinding to break it into smaller particle sizes (U.S. Pat. No. 7,654,477, U.S. Pat. No. 7,816,446, U.S. Pat. No. 8,013,036, and U.S. Pat. No. 7,445,170) are known. However, the size distribution reported in each of these patents is broad, ranging from about 75-1000 μm, and does not render the material appropriate for use as a pigment. Moreover, these patents do not provide a means for removing inclusions, such as steel and glass wool, from the ground material.
Crumb rubber made by cryogenically cooling the feedstock and grinding can produce a final product that can pass through a 140 mesh sieve, corresponding to particles <100 μm (U.S. Pat. No. 7,445,170). However, when such crumb rubber is included in a coating, the coating has a rough appearance, has low hiding power, is less black and has low tint strength. This combination of properties does not render the material appropriate for use as a pigment.